1. Field of the Invention
The present invention relates generally to communication systems. Particularly, the present invention relates to reverse link signal acquisition in a CDMA environment.
2. Description of the Related Art
The radio frequency (RF) spectrum is a limited commodity. Only a small portion of the spectrum can be assigned to each communications industry. The assigned spectrum, therefore, must be used efficiently in order to allow as many frequency users as possible to have access to the spectrum.
Multiple access modulation techniques are some of the most efficient techniques for utilizing the RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA modulation employs a spectrum technique for the transmission of information. The spread spectrum system uses a modulation technique that spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal. A signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz.
CDMA communications systems typically use directional antennas located in the center of a cell and broadcasting into sectors of the cell. The cells are located in major metropolitan areas, along highways, and along train tracks to allow consumers to communicate both at home and while traveling.
All of the mobile telephones communicating in the CDMA system transmit on the same frequency. Therefore, in order for the base station to identify each mobile, each mobile is assigned a unique pseudorandom (PN) spreading code that identifies that particular mobile to the system.
The mobile begins the registration process with a CDMA system by sending out a preamble signal that is comprised of chips. A base station searches for the preamble to determine if a mobile station is trying to communicate with the system. The base station may have to integrate over thousands of chips to find the mobile's preamble signal. This is typically not a problem if the preamble signal is at the proper frequency being searched by the base station.
Due to the Doppler effect, the critical cells are the cells located near highways or railway tracks. If a mobile is approaching a base station, the Doppler effect increases the signal's frequency as observed by the base station. If the mobile is moving away from the base station, the base station observes a signal having a frequency that is less than the frequency transmitted by the mobile. The amount of frequency shift is a function of the speed of the mobile.
The frequency of a signal transmitted by the mobile is aligned with the local oscillator in the mobile. The base station's frequency is synchronized with the Global Positioning System. When the mobile acquires a signal from the base station, that signal's frequency will be off. The mobile uses this shifted frequency to adjust its local oscillator in order to transmit back on the same frequency it has received. The base station then receives a signal that is shifted again by the Doppler effect. The base station is therefore receiving a signal that has double the frequency error. The two-way Doppler offsets may be in the range from 420 Hz for highway traffic to 1200 Hz for high-speed trains.
A typical frequency searcher experiences difficulty finding the signals transmitted by mobile telephones due to the double Doppler effect. The double Doppler effect may reduce a mobile's signal as much as 24 db below the threshold used by the searcher to find mobile signals. There is a resulting need for a frequency searcher that is capable of acquiring mobile signals that are affected by Doppler shift.